Bible Black, I mean it's the only commercial hentai I've watched and it's "weird"
I think it also was the first anime I've ever watched explicitly calling it as such.
I'm not sure if I
stole acquired it from the internet myself or swiped it from a friend's allegedly broken external drive he gave me to repair
(Some consider AN ANIME CALLED "BOKU" to be hentai, also watched that (episode 2) and frankly I don't consider it to be intended for sexual entertainment any more than than Goatse or Lemonparty are)